1. Field of the Invention
The invention relates to a gravity poured foundry mold and, in particular, to a gating system for such a mold.
2. Discussion of the Background
In common foundry practice, the flow of metal in a gravity poured mold is controlled by a gating system so as to produce one or more castings. Once the mold is full, the gating system, if properly designed, promotes good directional solidification of the casting or castings produced with that mold. Such molds may be made, for example, of sand, of graphite, or of a metallic-based material. They also may be based on any other material or combination thereof, so long as the metal being poured is compatible with the refractoriness of those materials and with the refractoriness of other materials used in the mold.
It has been known to use a skin-strainer in the gating system of a mold. Such skin-strainers are disclosed generically and in several specific embodiments in U.S. Pat. No. 4,154,289, the disclosure of which is hereby incorporated by reference.
FIG. 1 shows a mold in its pouring position comprising one mold casting cavity 1, and an embodiment of a skin-strainer 2 located somewhere along the top 3 of the mold casting cavity surface and preferably being located at those points requiring a traditional riser or feeder. Above the skin-strainer 2 is a conduit 5 which, in the illustrated embodiment, is a sprue-feeder extending to the top surface 4 of the mold. The sprue-feeder 5 communicates with mold casting cavity 1 through apertures 6 in skin-strainer 2, the apertures 6 collectively comprising the total passage cross-sectional area of the skin-strainer.
Although different molding practices may be used according to which a skin-strainer 2 may be located along the bottom or side surfaces of the mold casting cavity, the most natural and advantageous location is the surface of the casting cavity 1 which is on top when the mold is in pouring position. The skin-strainer 2 is firmly held in position at the lower end of the sprue-feeder 5 using retaining means 7 embedded in the top of part 8 of the mold. In the particular case of FIG. 1, the top of part 8 of the mold is made of bonded sand, the skin-strainer 2 is ceramic and the retaining means 7 is metallic. Other materials may be used, provided the refractoriness of those materials is compatible with the temperature of the metal being poured.
To fill the mold casting cavity 1, melted metal is poured directly into the sprue-feeder 5, first at a rate which is sufficient to quickly fill a major portion of the sprue-feeder, thereby choking the sprue-feeder, and then at a lower rate which is sufficient to keep the level of metal in the sprue-feeder substantially constant while the mold casting cavity 1 is being filled through the apertures 6. As shown, the apertures 6 may advantageously have tapered side walls.
A mold having a gating system comprising one or more skin-strainers 2 generally allows for the production of good quality castings at a lower cost than other traditional gating systems. However, choosing the size of the apertures 6 and the total passage cross-sectional area of the skin-strainer presents a conflict. On the one hand, the apertures must be sufficiently small to allow for easy choking of the sprue-feeder during pouring and to avoid the passing of melt inclusions through the apertures 6 into the mold casting cavity. On the other hand, the passage area must be large enough to allow not only for good filling of the cavity but also, and when necessary, for good feeding of the casting during its solidification.